Thermal cutoff switch



1967 A. J. NIELSEN, JR, ETAL 3,297,847

I THERMAL CUTOFF SWITCH Filed. Feb. 2, 1966 2 Sheets-Sheet 1 INVENTORS Anker J. N ielsen Jr. BY Lynn H. Morse Ja 10,1967 A. J. NIELSEN, JR., ETAL 3,

THERMAL CUTOF'F SWITCH Filed Feb. 2, 1966 2 Sheets-Sheet 2 INVENTORS Anker J. Nielsen Jr. BY Lynn H. Morse Maw ATTO EY United States Patent 3,297,847 THERMAL cU'roFF SWITCH Anker J. Nielsen,.lr., Holden, Mass, and Lynn H. Morse,

This invention relates to a thermal cutoff switch and, more particularly, to apparatus arranged to interrupt an electrical circuit in the event of a fire.

There are many instances in industrial and domestic electrical facilities where it is necessary to provide a means of interrupting the circuit in the event of excessive heat or fire. The most common use for such equipment is in connection with oil-fired domestic boilers where the local electrical codes require a thermally-responsive cutout for the oil burner circuitry for the ignition of the oil burner and the actuation of the pump. Devices which have been evolved in the past, however, have had many drawbacks. They have been complicated and expensive. Devices of the past have suffered from the defect that, when the high temperature situation causes the switch to open, it is possible for the home owner to re-activate the switch himself without investigating to see if anything is basically wrong with the circuitry. These and other difficulties experienced with the prior art devices have been obviated in a novel manner by the present invention.

It is, therefore, an outstanding object of the invention to provide a thermal cutoff switch which is effective to interrupt an electrical circuit, even though the equipment has remained untouched for many years.

Another object of this invention is the provision of a thermal cutoff switch in which the lead wires are used as the electrical switching contacts.

A further object of the present invention is the provision of a thermal cutoff switch in which the action of assembling the apparatus brings about a cleaning of the electrical contacts.

It is another object of the instant invention to provide a thermal cutoff switch in which there are no exposed wires or electrical surfaces.

It is a further object of the invention to provide a thermal cutoff switch which is formed as a unitary structure with the cover of an outlet box and in which tampering With the switch elements is impossible.

A still further object of this invention is the provision of a thermal cutoff switch which, when operated by heat to open an electrical circuit, cannot be re-assembled and made operative again.

It is a still further object of the present invention to provide a thermal cutoff switch which makes excellent use of modern plastic molding techniques to produce a compact, neat-appearing apparatus.

Another object of the invention is the provision of a thermal cutoff switch which is simple in construction, which is inexpensive to manufacture, and which is capable of a long life of useful service 'with no maintenance.

Another object of the invention is the provision of a thermal cutoff switch in which the electrical contact area between the elements is quite extensive and the area through which electrical current passes is not limited to point contact.

Another object of the invention is the provision of a thermal cutoff switch so constructed that the same high contact pressure which gives good electrical conduction also provides for positive mechanical locking of the parts.

With these and other objects in view, as will be apparent to those skilled in the art, the invention resides in the combination of parts set forth in the specification and covered by the claims appended hereto.

3,297,847 Patented Jan. 10, 1967 The character of the invention, however, may be best understood by reference to one of its structural forms as illustrated by the accompanying drawings in which:

FIG. 1 is a perspective View of a thermal cutoff switch embodying the principles of the present invention,

FIG. 2 is an exploded view of the switch,

FIG. 3 is a vertical sectional view of the switch taken on the line IIIIII of FIG. 1, and

FIG. 4 is a perspective view of a portion of the apparatus.

Referring first to FIG. 1, wherein are best shown the general features of the invention, the thermal cutoff switch, indicated generally by the reference numeral 10, is shown in use with an outlet box 11 having a cover 12. Extending from the top of the switch are two electrical lead wires 13 and 14 which are joined by connectors 15 and 16 to wires 17 and 18 forming part of an electrical circuit (not shown). The switch 10 is provided with a main body 19 from which extends a sleeve 21 having at its lower end a cap 22.

Referring to FIGS. 2 and 3, it can be seen that the main body 19 is formed by an upper portion 23 and a lower portion 24. The cover 12 is provided at its outer periphery with a slot 25 and diametrically opposite adjacent its periphery it is provided with a keyhole 26, the slot and keyhole permitting it to be bolted to the lower edge of the outlet box 11. The cover 12 is provided at its center with an aperture 27 which is in the shape of a circle having diametrically opposite flat portions, such as the flat portion 28. The upper portion 23 of the main body is provided with a raised portion 29 which exactly fits in the aperture 27 but has a thickness capable of extending only half-way through the cover 12. The lower portion 24 of the main body is provided on the end which faces toward the upper portion 23 with a similar raised portion 31 (see FIG. 3). These raised portions meet mid-way through the thickness of the cover, as is evident in FIG. 3. The upper portion 23 is provided at diametrically opposite portions with axial bores 32 and 33, while the lower portion 24 is similarly provided with bores 34 and 35, these bores to be placed in alignment and held together by rivets 36 and 37. The lower portion 24 is provided with a bore 38 which matches a recess 39 formed in the upper portion 23. The mating surfaces of the upper portion 23 and the lower portion 24 (where the raised portions 29 and 31 come together) are provided with transverse grooves 41 and 42. The lead wire 13 extends through an axial bore 43 in the upper portion 23 and its lower end is bent at a right angle to form a contactor 44 which fits exactly into the groove 41 with a portion of its surface extending into the passage 38 and the recess 39, as is evident in FIG. 3. Similarly, the lead wire 14 extends through an axial bore 45 in the upper portion 23 of the main body and has its lower end stripped and bent at a right angle to form a contactor 46 which lies in the grooves 42 and which also extends into the passage 33 and the recess 39. Extending into the lower end of the lower portion 24 of the main body and coaxial with the bore 38 is a somewhat larger counterbore 47 which has a cross section of a circular cylinder with two flat sides, as is evident in the drawing. Into this recess extends a switch element 48 which has a molded plastic body with a stem 49 in the shape of a cylinder having diametrically opposite flat sides to fit exactly and slidably into the counterbore 47. At its lower end, the body has an enlarged head 51. Molded into the plastic body of the switch element 48 and extending upwardly therefrom is a resilient junction member 52. The junction member is formed of strip metal in a generally U-form having its bight molded into the main body of the switch element 48. The two legs -of the junction member extend upwardly from the top of the body of the switch element in a somewhat outwardly spreading relationship, are inwardly dented to fit around the contac-tors 44 and 46, and then come together again.

The bottom end of the lower portion 24 of the main body is provided with a counterbore 53 into which fits the upper end of the sleeve 21. This upper end is swaged inwardly to form an inwardly-directed lip 54. Enclosed within the sleeve, pressing upwardly against the lip 54, and pressing downwardly against the upper surface of the head 51 of the switch element 48 is a coil spring 55. The bottom end coil of the spring 55 is locked in a suitable groove 56 formed at the junction of the stem 49 and the head 51. This serves to lock the spring 55 and the switch element 48 together securely. Finally, between the inner surface of the cap 22 and the outer surface of the lower end of the sleeve 21 is inserted a layer 57 of heat-fusible material.

Referring now to FIG. 4, the junction member 52 is formed from a strip of metal in a generally U-shape having a bight 58 and two legs 59 and 61. The legs have elongated lower portions 62 and 63 which diverge from one another slightly. The legs have relatively short upper portions 64 and 65 which are inclined inwardly so that the extreme ends of the legs touch. At the junction point between the lower portion 62 and the upper portion 64 of the leg 59 the metal is bent inwardly to form a transverse groove 66 of generally V-shaped conformation. A similar groove 67 is located on the leg 61 between the lower portion 63 and the upper portion 65.

In a practical embodiment of the invention, the main body 19, including the upper portion 23 and the lower portion 24, is formed of injection-molded plastic, such as a phenolic resin. The body portion of the switch element 48 including the stem 49 and the head 51 are also made of this same material, basically having a high insulating factor. The lead wires 13 and 14, of course, have an internal wire of copper covered with a plastic insulation, the insulation being stripped off in the vicinity of the contactors 44 and 46. The main body 21 and the cap 22 are formed of brass, while the spring 55 is formed from resilient wire. The junction member 52 is formed from copper strip which has been heat-treated and plated with silver. Finally, the layer 57 of heat-fusible material is formed of a fusible solder which melts at 165 F. It contains 42.50% of bismuth, 37.70% of lead, 11.30% of tin, and 8.50% of cadmium by weight.

The operation of the invention will now be readily understood, in view of the above description. As is evident from the drawings, the entire thermal cutoff switch is as a unit with the cover 12; the entire combination is sold as a unit. The leads 17 and 18, forming part of an electrical control circuit, are brought down into the outlet box 11 and are joined by the connectors 15 and 16 to the lead wires 13 and 14. The circuit becomes complete and operative when the junction member 52 is inserted between the contacts 44 and 46. The unit, including the cap 22, the sleeve 19, the spring 55, and the switch element 48 all are free during the assembly and are inserted by the electrician. He pushes the resilient junction member 52 up through the counterbore 47, through the bore 38 until the upper portions 64 and 65 of the junction member 52 come between the contacts 44 and 46. As he continues to press upwardly, the two legs are resiliently pressed toward one another, the free ends of the legs pressing together to form a completion of the electric circuitry. Finally, with considerably more force, he forces the two legs between the contacts 44 and 46 until they snap into place in the grooves 66 and 67. Since the contacts 44 and 46 have cylindrical surfaces and since the grooves 66 and 67 have V-shaped surfaces, they fit together very nicely and give a broad area of contact for electrical transfer.

The apparatus is now ready for use. The thermal cutoff switch 10 may remain in the'outlet box 11 for years without any call for duty. When a fire takes place, so

that extreme'heat is brought to bear against the cap 22 and the sleeve 21, the layer 57 of heat-fusible material melts at prescribed temperature. This causes separation of the cap 22 from the sleeve 21. The spring 55 is allowed to expand pressing between the head 51 of the switch element 48 and the lip 54 at the top of the sleeve 21. This pressure not only throws the cap 22 clear of the sleeve 21 with extremely high force, because of the considerable compressive strength of the spring 55, but it continues to press between the lip 54 and the head 51 and throws the switch element 43 outwardly of the main body 19. The junction member 52 is jerked suddenly from between the contacts 44 and 46 and the circuit is broken. The electrical circuitry connected to the leads 17 and 18 is no longer operative. I

If the electrical circuitry is interrupted and the home owner proceeds to the cellar and sees the cap 22, the sleeve 21, the spring 55, and the switch element 48 as well as the junction member 52 lying on the floor, he may attempt to place them back in operation to re-energize the circuit again. He might try to place the cap 22 on the end of the sleeve 21 and then to press the entire assembly back into the main body 19. He is able to close the circui-t by inserting the junction member 52 between the contacts 44 and 46, but if he attempts to leave, the spring 55 will immediately eject the cap 22 from the end of the sleeve and pull the switch element 48 out of the main housing again. He is, therefore, not able to put the electrical circuitry in operation. The portion of the invention which makes this possible has to do with the spring 55 looking into the groove 56 at the base of the plastic portion of the switch element 48. If the spring were not fastened to the stem, he could probably place the apparatus back in operation. The spring causes everything to be rendered inoperative the moment he releases his finger from the cap 22 in trying to replace the elements.

Some of the advantages obtained by the present construction lie in the fact that the lead wires Band 14 are used as the fixed contacts in the switch. This removes the difficulty of having multiple connections within the switch itself and does away with the possibility of incomplete connections within the switch. The novel manner in which the parts of the main housing 19 are held together by the rivets 36 in conjunction with the cover 12 assures that all of the elements are in good operating condition and that a good. dust-tight seal is provided around the switch and around the aperture 27 in the center of the cover. It should be noted that the action of introducing the junction 52 between the contacts 44 and 46 causes a Wiping action on the contacts just before the actual electrical contact is made in the V-shaped grooves 66 and 67. This assures that good electrical continuity exists between the two contacts 44 and 46. The current not only passes down the legs of the resilient junction member 52 through the bight 58 to the other side, but also goes up through the upper portions 64 and 65 where the ends are pressed together in the upper portion. There are, therefore, two complete circuits through the junction member. As has been mentioned before, the contacts between the contactors 44 and 46 and the V-shaped grooves 66 and 67 are in the nature of broad area contact rather than point contact, as has been true in many of the prior art devices of this kind.

In inserting the resilient junction member 52 between the contactors, it should be noted that a peculiar action takes place. First of all, in making this insertion, there is a soft spring action between the two leaves of the junction member which permits ready introduction of the element between the contactors. However, when the points of the contactor of the junction member come together, then the spring action gets stiffer and brings about a stiff contact pressure. In other words, the pressure is easy at first for admission and then hard for final electrical contact.

Of course, the electrical load-carrying capacity is increased at the time that the ends of the two legs of the junction member are pressed tightly together.

There is, of course, a complete insulation for exposed surfaces of the wires since the exposed portions (the contactors 44 and 46) are completely surrounded by plastic, assuming that the grooves 41 and 42 are made of a suitable depth and form. Locating dowels are provided on the two halves of the main body 19 to assure that the two members are in exact proper alignment and, of course, it is impossible to put the plug element, including the cap 22, the sleeve 21, the spring 55, and the switch element 48 into the switch in the wrong angular relationship, since the counterbore 47 and the stem 49 are non-circular and of the same size.

It is obvious that minor changes may be made in the form and construction of the invention without departing from the material spirit thereof. It is not, however, desired to confine the invention of the exact form herein shown and described, but it is desired to include all such as properly come within the scope claimed.

The invention having been thus described, what is claimed as new and desired to secure by Letters Patent 1s:

1. A thermal cutoff switch, comprising (a) a main body,

(b) a pair of spaced electrical contacts mounted on said body,

(0) a switch element removably inserted in said body,

(d) a resilient junction member mounted on said switch element for joining said contacts by being pressed between them,

(e) a heat-fusible means interconnecting said body and said switch element,

(t) spring means mounted on said body for biasing said switch element and resilient junction member away from said body and contacts when said heatfusible means is melted, and

(g) means locking said spring means and said switch element together to prevent re-assembly of said switch element with said body once said heat-fusible means has melted.

2. A thermal cutoff switch as recited in claim 1, wherein the element consists of a body formed of plastic in which the junction member is molded, the junction member comprising a strip of sheet metal bent to U- shape with the bight embedded in the plastic body, the legs of the junction member extending generally straight and parallel to each other from the plastic body and being inclined inwardly toward each other at the outer ends, a transverse groove being formed on each leg at the junction of the straight and inclined portions to receive a contact.

3. A thermal cutoff switch as recited in claim 1, wherein the main body consists of two separable portions each molded of plastic, the portions being formed to be fastened together in a non-circular aperture in the cover of an outlet box, the adjacent ends of the portions having raised portions that exactly fit in the said aperture.

4. A thermal cutoff switch as recited in claim 1, wherein the main body is molded of plastic and has a passage in which the resilient junction member lies, the contacts consisting of rod-like members lying in the passage and extending transversely and on opposite sides thereof.

5. A thermal cutoff switch as recited in claim 1, wherein the body is provided with a metal sleeve having one free end, wherein the said means comprises a metal cap slidably mounted on the free end of the sleeve, and wherein the cap and sleeve are fastened together only by a heat-fusible substance.

6. A thermal cutoff switch as recited in claim 5, wherein the spring is a coil spring lying within the sleeve coaxially thereof, wherein the said element comprises a plastic rod lying within and coaxial of the sleeve and having an enlarged head, and wherein the spring is nor mally compressed between the enlarged head of the rod and the end of the sleeve opposite the free end.

7. A thermal cutoff switch as recited in claim 1, wherein the junction member is formed to give a slight amount of lateral pressure during insertion between the contacts and a large amount of pressure at completion of the insertion.

8. A thermal cutoff switch, comprising (a) a main body,

(b) a pair of spaced electrical contacts mounted on said body,

(0) a switch element removably inserted in said body,

(d) a resilient junction member mounted on said switch element for joining said contacts by being pressed between them,

(e) a heat-fusible means interconnecting said body and said switch element,

(f) spring means mounted on said body for biasing said switch element and resilient junction member away from said body and contacts when said heatfusible means is melted, and

(g) two electrical lead wires which extend into the body and are bent at their inner ends at right angles, which inner ends constitute the said contacts.

9. A thermal cutoff switch as recited in claim 8, wherein the body is formed in two parts with mating surfaces, the mating surfaces being formed with matching grooves to hold the said inner ends of the lead wires.

No references cited.

BERNARD A. GILHEANY, Primary Examiner. H. B. GILSON, Assistant Examiner. 

1. A THERMAL CUTOFF SWITCH, COMPRISING (A) A MAIN BODY, (B) A PAIR OF SPACED ELECTRICAL CONTACTS MOUNTED ON SAID BODY, (C) A SWITCH ELEMENT REMOVABLY INSERTED IN SAID BODY, (D) A RESILIENT JUNCTION MEMBER MOUNTED ON SAID SWITCH ELEMENT FOR JOINING SAID CONTACTS BY BEING PRESSED BETWEEN THEM, (E) A HEAT-FUSIBLE MEANS INTERCONNECTING SAID BODY AND SAID SWITCH ELEMENT, (F) SPRING MEANS MOUNTED ON SAID BODY FOR BIASING SAID SWITCH ELEMENT AND RESILIENT JUNCTION MEMBER AWAY FROM SAID BODY AND CONTACTS WHEN SAID HEATFUSIBLE MEANS IS MELTED, AND (G) MEANS LOCKING SAID SPRING MEANS AND SAID SWITCH ELEMENT TOGETHER TO PREVENT RE-ASSEMBLY OF SAID SWITCH ELEMENT WITH SAID BODY ONCE SAID HEAT-FUSIBLE MEANS HAS MELTED. 